Optical image scanning and reading devices (image reading devices) read indicia, such as barcodes, that represent data about a product or service. A barcode is an optical machine-readable label attached to an object, which directly or indirectly represents information about the object or a service associated with the object. Such information can include, without limitation, vendor identification, product name, price, patient name and other descriptive information about the object. Barcode reading devices are widely used in distribution, retail and many other industries for reading barcodes.
Often, such devices are based upon charge coupled device (CCD) or CMOS technology, wherein a linear array CCD or CMOS device is used to recover light reflected from the barcode. In such systems, plural LEDs are typically used as a light source to illuminate an object such as a barcode. The reflected light is received by the CCD or CMOS linear array, which converts the light energy into electrical energy. The varying electrical signal can then be processed to recover an image of the object, which represents the information of interest.
Image readers are used for reading many different types of barcodes and other optically readable labels and indicia. For example, an image reader can be used for reading an electrical shelf labels as shown in FIG. 10A. The electrical shelf label includes a bar code label printed on paper and a reflective liquid crystal. The liquid crystal typically has a lower reflectivity than the printed label. Therefore, if the illumination period of the image reader is set relatively short with a low-intensity level of illumination to optimize the exposure of the printed label, the liquid crystal will be significantly under-exposed as shown in FIG. 10B. If, however, the illumination period of the image reader is extended to optimize the exposure of the liquid crystal, then the printed label will be significantly over-exposed as shown in FIG. 10C.
Image readers are also used for reading barcodes displayed on liquid crystal displays (FIG. 10D) and bar codes directly marked on surfaces of various types of objects and materials, such as a circuit board (FIG. 10E) or a metal surface (FIG. 10F). Similar to the liquid crystal applications, the illumination period of the image reader must be extended to optimize the exposure of such barcode images due to their low reflectivity.
In other applications, the barcode is moving when the image reader captures an image of it, as shown in FIG. 10G. If an extended illumination period (with low-intensity level of illumination) is continuously applied as the barcode moves across the field of view of the image reader, the image of the barcode will be blurred, as shown in FIG. 10H. Therefore, a very short illumination period with a high-intensity level of illumination is used to obtain a sharp image of the moving barcode, as shown in FIG. 10I.
In still other applications, the image reader may capture an image of a barcode in an environment with very low ambient lighting, such as a dark warehouse and or if the bar code is at a far distance from the image reader. As shown in the graph of FIG. 10J, the illumination intensity decreases with increasing depth of field or distance from the image reader. If a very short, pulsed illumination period (with a high-intensity level of illumination) is applied to read the barcode, the image of the barcode will be under-exposed, as shown in FIG. 10K. Therefore, an extended illumination period with a low-intensity level of illumination is used to obtain an optimally exposed image of the barcode, as shown in FIG. 10L. The low-intensity level of illumination is used because the illumination sources (e.g., LEDs) generate an excessive amount of heat when driven by a large current to generate the high-intensity level of illumination. Therefore, if the illumination sources are driven for an extended illumination period to produce a high-intensity level of illumination, the life time and reliability of the illumination sources can be negatively affected.
Accordingly, an image reader is needed which is capable of capturing images of moving indicia, indicia in dark environments, indicia with low reflectivity and any combination thereof, with optimal exposure and without negatively affecting the life time and reliability of the illumination sources of the image reader.